One of the principal problems of power supplies for gaseous discharge lamps is heat produced in the semiconductors and other related components, for example transformer windings. Many different configurations are known for use in power oscillators and converters to achieve better efficiency and less heat dissipation. There are advantages and disadvantages to some of the most widely used configurations.
The most widely used configuration is two transistors in a push-pull arrangement oscillating in counter phase. This configuration presents the problem that with a 120 VAC supply and a full wave bridge rectifier, the filtered DC will be 170 volts, and the collector to emitter voltage of the transistor in the off condition goes up to 340 VDC. This requires a transistor with an E.sub.CBO of 340 VDC plus 20% for a safety factor, totaling 408 VDC. A transistor with this parameter and high current and gain will be relatively expensive.
Electronic designers, in trying to achieve a more economical configuration, have produced full and half bridge converters. The full bridge configuration uses four transistors connected in series, two by two across the full voltage of the power supply, with the primary of the transformer connected across the common emitter-collector point of each of the two series transistors. Using the voltage of the push-pull configuration described above as a comparison, the total reverse voltage E.sub.CBO across the transistors is now 170 VDC, which is the voltage of the power supply. The transistors divide the power supply voltage, and the potential applied to the primary winding is now half of the source voltage. However, there is a drawback. The collector current needs to be doubled in order to transmit the same power, and four transistors are required to implement the circuit with a correspondingly more complicated base polarization network.
One circuit that is very popular is derived from the full bridge. Electronic engineers looking for simpler design developed the half bridge. Two of the transistors are replaced by two capacitors, forming a capacitive voltage divider. The only advantage of this configuration is the saving of two power transistors. Neither the full nor the half bridge is a self starting oscillator. Both need to be driven by special circuitry which is electrically isolated, and the higher currents demanded by the powered circuits pose an additional stress to the power transistors.
Another very popular configuration, due to its simplicity, is the one transistor blocking oscillator used extensively in flyback circuits. Its major disadvantage is the limitation of power handling capability, on the order of 50 watts. Higher powers are possible, but the circuit becomes too complicated, and the initial simplicity is lost.
Finally, complementary symmetry is sometimes employed in such power supply circuits. The implementation of complementary symmetry requires a pair of matched PNP-NPN transistors, which is one of the causes of its lack of popularity.
One of the major problems encountered in solid state power converters is long transistor turn off time. Bipolar transistors have a minority carrier stored base charge, and this makes them slower. The common base capacitance establishes their switching characteristics. This makes their storage time longer, and a large collector current is difficult to cut off in a short time. When the collector current is controlled by the inductance of the primary, as is the case in all the configurations cited above, at the end of the conduction time, the inductance value is smaller than at the beginning. Even in driven transformers where the operating B value (magnetic flux density) of the core is chosen far away from saturation, the inductive reactance is much smaller at the end than in the beginning of the conduction time. This inherent fact makes the current density heavier at the end of the cycle and more difficult to turn off. The storage time of the transistor, at this particular moment, becomes longer and the transistors start to have simultaneous conduction at the mutual turn off/turn on time. The delay in turn off can be as long as two microseconds. Particularly at this moment when the two transistors conduct simultaneously, the value of the inductance drops to zero and a very short transient of high collector current is produced. The ohmic resistance of the related circuits limits the current. This situation worsens when there is a high collector current just before the turn off time. This causes overheating of the transistor junction and its eventual destruction.
There are electronic tricks to speed up the turn off time of power transistors. They are implemented by extensive use of capacitor, diode, and resistor combinations all of kinds. The problem is so serious that the major semiconductor companies developed various kinds of pulse width modulator (PWM) circuits for specific application in switching power supplies.
The maximum current through an inductance occurs at the end of five time constant periods. At this time, the current will be the maximum allowed by the circuit. In contrast to this, when a capacitor charges or discharges, at the end of the five time constant periods, the current will be virtually zero. The capacitor current before the fifth time constant will always be smaller than the current at the beginning of the conduction time. If the frequency and the value of the capacitor are selected in such a way that the conduction time is longer than one time constant period, the current at the end of the conduction time will be less than 36.8% of the initial value. The use of this particularity will ease the turn off of a power transistor. If two constant times are achieved, the value of the current will be 13.5% of the initial value.